Background radiation from alpha particles, neutrons and cosmic rays can create momentary upsets (so-called single-event upsets, or SEUs) in data inside an integrated circuit (IC). Some SEUs, called single-event transients (SETs), do not affect bit values. Other, more severe SEUs may affect the value of one or more bits. SEUs that affect the value of one bit are called SBUs. The rate at which SBUs occur affects the IC's soft error rate (SER). SBUs may go unnoticed if the data is changed back to the correct value before it is stored. However, an SBU may cause an error if the upset data is stored or if the upset directly changes the data contained in a storage element. Decreases in feature sizes and operating voltages have caused the SER of standard logic elements in ICs to rise. One of these standard logic elements is a DFF.
As those skilled in the pertinent art are aware, DFFs have a data output, Q, that always assumes the state of a data input, D, when a clock signal provided to the DFF goes high. In other words, until an edge occurs in the clock signal, Q maintains the state D had at the last occurrence of the same edge-type. In this sense, DFFs act as a temporary storage element or delay line. These are basic functions. Consequently, DFFs find wide use in shift registers and other logic circuits. Any vulnerability a DFF may have to SBUs can be of major concern.
Some DFFs are specially designed to function in high radiation environments. They use mitigation schemes such as temporal sampling to provide SBU immunity. Temporal sampling employs multiple (typically three) storage elements for each bit of data and a voting scheme to overrule an SBU-affected storage element. Unfortunately, temporal sampling not only causes DFFs to be significantly larger (stemming from the multiple storage elements required), but also slows down their operation, reducing the performance of any IC using such DFFs.
Most DFFs are not specially designed to function in a high radiation environment. Ordinary DFFs are either sufficiently large or operate at a higher voltage such that the SBU problem is not as pronounced or include no mechanism to address the SBU problem whatsoever and suffer the consequences. Unfortunately, applications in which SBU immunity is not absolutely critical may still have a high cost associated with SBUs. Those applications may benefit from a DFF that has a decreased vulnerability to SBUs without resorting to elaborate mitigation schemes such as temporal sampling. Standard libraries of logic elements including DFFs may also benefit from such an enhanced-immunity DFF. Those applications and standard libraries may further benefit from latches that exhibit an enhanced immunity to SBUs.